The tracings on the EKG paper are a reflection of electrical activity of the heart. A series of body surface electrodes are placed at specific points on the arms, legs and thorax that sense and record the heart's electrical activity. The electrodes are assigned a specific polarity - i.e. - either negative or positive.

For any lead, the EKG machine looks at specific combinations of leads in order to configure the tracing we expect to see for Lead I, II, III, aVF, aVL, aVR or for V1-V6. The electrode that we care about the most is called the sensing electrode which is always given a positive polarity (+). Hence, we call it the positive sensing electrode.

In order to understand what the EKG tracing is saying to us, there are a few general rules to remember for depolarization and repolarization.

Rules of Depolarization : Making Sense Out Of Apparent Chaos - Part I

General Rule # 1 For Depolarization

If a wave of depolarization is generally moving toward the positive sensing electrode, the EKG tracing will record a positive deflection above the isoelectric line on the EKG paper

General Rule # 2 For Depolarization

If the wave of depolarization is generally moving away from the positive sensing electrode, the EKG tracing will record a negative deflection below the isoelectric line on the EKG paper.

General Rule # 3 For Depolarization

If the wave of depolarization moves perpendicular to the line of sight of the positive sensing electrode, the EKG tracing will record a biphasic tracing on the EKG paper.

Rules of Repolarization : Making Sense Out Of Apparent Chaos - Part II

General Rule # 1 For Repolarization

If a wave of repolarization is generally moving toward the positive sensing electrode, then the EKG tracing will record a negative deflection below the isoelectric line on the EKG paper.

General Rule # 2 For Repolarization

If a wave of repolarization is generally moving away from the positive sensing electrode, then the EKG tracing will record a positive deflection above the isoelectric line on the EKG paper.

General Principle # 3 For Repolarization

If a wave of repolarization moves perpendicular to the line of sight of the positive sensing electrode, then the EKG tracing will record a biphasic tracing on the EKG paper.

Time, Intervals, And Amplitude Of The EKG Tracing

The EKG paper is a ruled paper that is usually heat sensitive. The smallest division on the paper is a one millimeter box. The largest division on the paper is a five millimeter box. By international convention, the speed of the paper moves at a rate of 25 millimeters per second past the stylus. This is the speed that makes all of the tracings look appropriate for any lead that is being viewed. Because the speed of the paper is 25 mm/second, a small 1 mm box is traversed in .04 seconds and a large 5 mm box is traversed in .2 seconds.

Determination of Heart Rate

Because of the predictable EKG paper speed, we can count the number of boxes traversed over a period of time between heart beats to determine HR.

Methods For Determining Heart Rate From The EKG Strip

Method # 1 : The Cardiac Ruler Method

Place the beginning point of a cardiac ruler over an R wave. Look at the number on which the next R wave falls and that becomes the heart rate for that patient. Use the following numbers to indicate what the heart rate is between two successive R waves : 300, 150, 100, 75, 60, 50, 43, 37, 33, 30

Method # 2 - The Six Second Tracing MethodObtain a six second tracing (30 five mm boxes) and count the number of R waves that appear within that 6 second period and multiply by 10 to obtain the HR/min.

Example : If there are 12 five mm boxes in a 6 second tracing then the heart rate would be : 12 x 10 = 120 bpm.

Method # 3 - The 300 MethodCount the number of large boxes between 2 R waves and divide this number into 300 to obtain the HR/min.

Example : If there were 2.5 large 5 mm boxes between two successive R waves, then the heart rate would be : 300/2.5 large boxes = 120 bpm

Method # 4 - The 1500 MethodCount the number of small boxes between two R waves and divide this number into 1500 to obtain the HR/min.

Example : If there were 12.5 small boxes between two successive R waves, then the heart rate would be : 1500/12.5 small boxes = 120 bpm.

Amplitude or Voltage : Amplitude of the deflected wave is measured in millivolts (mV). The voltage of a wave deflected through one large 5 mm box is a deflection said to have a 0.5 mV value.

The Six Limb Leads Of The 12-Lead EKG
Three Standard Leads : Lead I, Lead II, Lead III
Three Augmented Leads : aVF, aVR, aVL

The six limb leads (Leads I, II, III, aVL, aVF, and aVR) look at the electrical output of the heart in the frontal plane. By international convention, a circle is drawn from the chin to the symphysis pubis to describe the area in which the limb leads view the heart. The circle is divided along the horizontal X axis and the verticle Y axis. The top half of this circle is designated to be wholely negative with the circle enumerated as 0 to -180 degrees. The bottom half of this circle is designated as wholely positive with the circle enumerated from 0 to +180 degrees. The circle is further divided into 90 degree halves such that the frontal plane is now cut into four equal 90 degree quadrants : the normal axis quadrant from 0 to + 90 degrees; the left axis deviation quadrant from 0 to -90 degrees; the right axis deviation quadrant from +90 to +180 degrees; and finally the extreme right axis deviation quadrant from -180 to -90 degrees.

How Are The Standard Leads Configured By The EKG Machine ?

1) Lead I is created by making the left arm positive (+) and the right arm negative (-). Its angle of orientation is + 0°. Lead I looks across the heart from the right side of the thorax to the left side of the thorax along the +0° axis in the frontal plane.

2) Lead II is created by making the left leg positive (+) and the right arm negative (-). Its angle of orientation is +60°. Lead II looks across the heart from the right shoulder down through the thoracic cage to the left hip along the + 60° axis in the frontal plane.

3) Lead III is created by making the left leg positive (+) and the left arm negative (-). Its angle of orientation is a +120°. Lead III looks across the heart from the left shoulder down through the thoracic cage to the right hip along the +120° axis in the frontal plane.

4) aVL is created by making the left arm positive (+) and the other extremities negative (-). Its angle of orientation is -30°. aVL looks across the heart from the right leg up through the thoracic cage to the left shoulder along the -30° axis in the frontal plane.

5) aVR is created by making the right arm positive (+) and all other extremities negative (-). Its angle of orientation is -150°. aVR looks across the heart from the left hip up through the thoracic cage to the right shoulder along the -150° axis in the frontal plane.

6) aVF is created by making the legs positive (+) and all other extremities negative (-). Its angle of orientation is +90°. aVF looks through the heart from the chin down through the thoracic cavity to the feet along the +90° axis in the frontal plane.

Groups Of Leads Look At Specific Sections Of The Heart

The leads looking at the inferior portion of the heart are : II, III, aVF.
The leads looking at the left lateral portion of the heart (left ventricle) are : I, aVL.
The augmented lead aVR looks toward the right side of the heart and is considered to be a lead in "no man’s land".